1. Field of the Invention
This invention relates to a partially "transient" boron additive for MCrAlY type basecoats which are deposited on superalloy, high temperature turbine substrates. The boron additive improves the density and coating quality of the basecoat.
2. Background Information
A cobalt or nickel based superalloy of, for example, the elements Ni.Cr.Al.Co.Ta.Mo.W, has been used for making blades, vanes, and other components for gas turbines. These turbine components are generally protected by a basecoat of MCrAlY, where M is selected from the group consisting of Fe, Co, Ni and their mixtures. These basecoats are usually covered by an oxidative overcoat layer and a final thermal barrier coating, as taught, for example, in U.S. Pat. Nos. 5,180,285; 5,562,998; and 5,683,825 (Lau, Strangman, and Bruce et al., respectively).
In some instances, a separate substrate contacting layer is used at the interface between the substrate and the MCrAlY basecoat. For example, an aluminide or platinum layer is mentioned as a separate substrate contacting layer to provide basecoat durability in U.S. Pat. No. 4,321,311 (Strangman).
All these turbine components operate in high temperature environments, and generally the higher the temperature the more efficiency can be realized, within materials limitations. One of these materials limitations is attachment of turbine components to each other, and attachment of the MCrAlY and other layers to the superalloy substrate of the turbine blade, or the like.
Bonding powders, including temperature depressants, selected from at least one of B, Si, Mn, and Ta, as well as precipitation strengthening elements, such as Al and Ti, and solution strengthening elements, such as Mo or W, have been added from 1 wt % to 15 wt % in nickel base superalloy compositions to allow ease of brazing turbine airfoils, and the like, to base portions at overlap and butt joints, as taught in U.S. Pat. No. 3,692,501 (Hoppin et al.). About 0.5 wt. % to 16 wt. % silicon has been added to a FeCrAlY type nitrocellulose slurry, for spray painting on nickel base superalloys, followed by a diffusion heat treatment. These compositions provide an adherent, oxidation resistant coating, as taught by U.S. Pat. Nos. 3,741,791 and 4,034,142 (Maxwell et al. and Hecht, respectively). In U.S. Pat. No. 5,316,866 (Goldman et al.) a Ni.Co.Cr.Al.Mo.Ta.W coating, also containing under 0.1 wt. % C,B, and Zr, was substituted for the standard MCrAlY composition, next to a nickel-based superalloy. Amounts of C over about 0.07 wt. % or B, or Zr over about 0.030 wt. % are taught as causing grain boundary embrittlement.
What is still needed, however, are denser, higher quality, less expensive MCrAlY type basecoats, which can be used without a separate thermal barrier layer. Protective MCrAlY base coats are still used as protection for turbine components. While these coatings have made significant technical contributions to the industry, they still suffer from high cost and variable quality. Some areas of turbine components, such as fillet regions, are particularly difficult to coat using standard MCrAlY type basecoats. Frequently the applied MCrAlY coating will contain excessive porosity, which can result in poor performance. What is also needed is an alternative coating process which provides exceptional bonding and high densification of the MCrAlY coating, providing increased performance and superior turbine component protection.